Colour separation, full colour printing, colour illustration, colour representation and colour manipulation are essential features of the invention. It is therefore of paramount importance that the specification and accompanying colour figures are interpreted against this background.
In printing processes, whether it be so-called conventional printing (e.g. flexographic, rotogravure, lithographic or silkscreen printing), or digital printing (e.g. inkjet, laser or toner printing), full colour printing onto a substrate is generally accomplished using four primary colours, namely Cyan, Magenta, Yellow and Black, which are collectively abbreviated in the trade as “CMYK”. These colours generally conform to an ISO standard specifying colour characteristics and tone value increase curves (“TVI”) for the respective inks.
To a lesser degree, printing in full colour can be accomplished using additional primary colours to increase a colour gamut. In this instance, CMYK may be enhanced by adding appropriate Orange, Green or Violet inks. A combination of Cyan, Magenta, Yellow, Black, Orange, Green and Violet inks is collectively abbreviated in the trade as “CMYKOGV”. CMYK may also be enhanced by adding appropriate Red or Blue inks, in which case a combination of Cyan, Magenta, Yellow, Black, Red, Green and Blue inks is collectively abbreviated in the trade as “CMYKRGB”. Other expanded or extended colour gamut processes may involve the use of other, but similar primaries to achieve gamut volume expansion.
Whether printing with CMYK or extended gamut processes, it is essential that these inks not only deliver the appropriate colour, but that they are also transparent or semi-transparent, since trapping (i.e. overprinting) of these primary colours allow for additional colours to appear. By way of example, printing Cyan over Yellow creates Green; printing Magenta over Yellow creates Red; printing Cyan over Magenta creates Blue; and so on. If only pure, transparent Cyan, pure transparent Magenta, and pure transparent Yellow were used, without applying tones, a maximum of seven apparent colours would be observable, including Black, if all colours were mixed together, as illustrated in the colour diagram of FIG. 13.
In practice, a dark black colour is difficult to achieve using only CMY and hence a Black ink is added. With both CMYK and CMYKOGV/CMYKRGB, or any other process ink system, transparent primary colours are used to create full colour images by printing different colours over each other. For these printing systems and processes to work, it is very important that a first printed ink colour properly dries before a next ink colour is deposited over the first ink colour. If the inks are not properly dried, trapping is poor, which means that one ink repels the other ink, leading to a lack of apparent, secondary trapping colours. Alternatively, wet inks contaminate each other and/or a printed substrate. Moreover, the requirement to trap inks when printing full colour images, may lead to use of an excessive amount of ink printed onto a substrate. Some substrates, such as newspaper or tissue paper, may not be tolerant of high ink coverage, which makes colour printing onto such substrates challenging.
In certain printing processes, such as Dry Offset Letterpress, an inked image is transferred (or “offset”) from a printing plate onto a rubber blanket, and from the rubber blanket onto a printing substrate surface. In these processes, all the inks are transferred onto the rubber blanket in a wet state before being transferred onto the substrate, and the inks are transferred onto the printed substrate simultaneously for the sake of accurate registration. Drying of the inks, once transferred onto the substrate, subsequently takes place under a UV light and/or heat source. Major problems associated with Dry Offset Letterpress printing are that, since the printing inks are deposited onto the rubber blanket in a wet state, they cross-contaminate each other, or simply don't transfer properly onto the blanket or substrate, or don't create the expected trapping colours.
It is known in the prior art to digitally separate images, objects or colours to be printed in full colour, into CMYK or CMYKOGV or CMYKRGB channels, each corresponding to an ink that will be used. This is typically done at a bit depth or bit level of 8 or more (greyscale). Bit depth or bit level can be explained as follows. Referring to the colour diagram above, by using only pure Cyan, Magenta and Yellow, a maximum of seven different colours can be created. However, if one were to mix 256 shades of Cyan, 256 shades of Magenta, and 256 shades of Yellow, one would be able to create 16.8 million different colours. That is exactly what you get with an 8-bit image for example—256 shades of red, 256 shades of green, and 256 shades of blue, which when trapping each other, collectively create millions of possible colours typically present in an 8-bit CMY(K) file on a computer.
The number 256 is mathematically arrived at as follows: 1-bit equals 2. Moving beyond 1-bit, bit value is calculated by the expression “2 to the exponent (however many bits there are)”. So, for example, the value of 2-bits is calculated as “2 to the exponent 2”, which equals 4. So 2-bits equals 4. A 4-bit image would be “2 to the exponent 4”, which is 16. So 4-bits equals 16. Therefore, an 8-bit image would be “2 to the exponent 8”, which is 256. This means that for, say the colour Cyan, a 1-bit image, when displayed without applying cyan colouring, would be a pure black-and-white image only having no shades of grey; whereas an 8-bit image would be a grayscale image comprising 256 shades of grey.
Explained alternatively, for each pixel in an image, greyscale tonal value or level of the pixel is stored in an electronic image file. Bit depth refers to how many digits the tonal information for each pixel is stored in. For a bit depth of 1, one digit would store how light or dark each aspect of a scene was, and since the only possible 2 values for a bit depth of 1 would be 0 and 1, and the only two tones that could be represented are black and white, as illustrated in the black-and-white image of FIG. 14A.
By comparison, a bit depth of 8 provides for 256 shades of grey, which means that the image could then be displayed as illustrated in the greyscale image of FIG. 14B.